One solution to this problem which has been used in such printers during the last 15 years has been a helical spring disposed in the slot and provided with eight evenly distributed plastic elements for example, to prevent the front edge of the document from entering the slot. The helical spring is suspended between the magnetic head and one end of the slot. In the home position of the magnetic head for feeding the document into or out of the printer, i.e. when the magnetic head is at the other end of the slot, the helical spring is at: its maximum extent and the spacing between the plastic elements is also at a maximum. When the magnetic head is at its other end position, the spring assumes its shortest length and the plastic elements are packed tightly together at one end of the slot, and this reduces the potential length of stroke of the magnetic head in the slot by about one third. This design has proved complicated and expensive. The mounting of the spring and the plastic elements spaced thereon is quite labour-consuming.
Another unsolved problem in this context has been to achieve a positive non-slipping but at the same time smooth reciprocal movement of the magnetic head. In the previous design described above with a helical spring and plastic elements, it was necessary to fix the drive belt to the lower portion of the magnetic head, and this gave rise to a certain amount of stick-slip movement.
An object of the present invention is to achieve a drive device of the type in question which solves these problems.